题目

When you hear of plastic surgery, what do you think of? A Hollywood star is trying to look younger than her real age? People who want to change the size of their stomachs, breasts, or other body parts because they see it done so easily on TV? Those are what people usually think of plastic surgery, but what about the 4-year-old boy who has his chin rebuilt after a dog bit him? Or the young woman who has the birthmark on her forehead lightened with a laser? Plastic surgery is a special type of surgery that can involve both a person's appearance and ability to function. Plastic surgeons t小明在做“探究摩擦力大小与哪些因素有关”的实验，若水平匀速拉动木块，弹簧测力计的示数为5N，由二力平衡知道木块受到摩擦力为5N；若加速拉动木块时，弹簧测力计的示数为8N，则此时摩擦力为________N．